Temsirolimus, an Inhibitor of Mammalian Target of Rapamycin Brian I

CCR Drug Update

Temsirolimus, an Inhibitor of Mammalian Target of Rapamycin Brian I. Rini

Temsirolimus (Torisel, Wyeth Pharmaceuticals) is an inhibitor growth inhibition, and this effect is mediated by hypoxia- of mammalian target of rapamycin (mTOR), a molecule impli- inducible factor (3). The second major mTOR effect is on the cated in multiple tumor-promoting intracellular signaling path- 4E-binding protein-1 and eukaryotic initiation factor-4 subunit ways. Temsirolimus recently became the first Food and Drug E complex. Activated mTOR phosphorylates 4E-binding pro- Administration–approved mTOR-targeted agent based on a tein-1, promoting dissociation of this complex and allowing phase III trial showing an overall survival advantage over IFN-a eukaryotic initiation factor-4 subunit E to stimulate an increase monotherapy in advanced renal cell carcinoma (RCC) patients in the translation of mRNAs that encode cell cycle regulators, with multiple adverse risk features (1). such as c-myc, cyclin D1, and ornithine decarboxylase. mTOR Temsirolimus is a soluble ester of rapamycin, a natural pro- inhibition results in a block of 4E-binding protein-1 phospho- duct that was initially developed as an antifungal drug and then rylation, sequestration of eukaryotic initiation factor-4 subunit as an immunosuppressive agent, with anticancer activity noted E, and failure to form the complex required for translation. more than 20 years ago. Rapamycin (sirolimus, Rapamune) Inhibition of mTOR by temsirolimus requires a specific bind- was isolated from the soil bacteria Streptomyces hygroscopicus ing complex. Temsirolimus forms this complex with the found on Rapa Nui (commonly known as Easter Island) in the FK506-binding protein and prohibits the activation of mTOR. South Pacific in 1975, but its development for cancer ther- This mechanism of action is similar for sirolimus, a temsir- apeutics was not prioritized. The immunosuppressant effects olimus metabolite, and both are likely responsible for of rapamycin were pursued and resulted in Food and Drug inhibition of mTOR and antitumor effects after administration Administration approval in 1999 for prevention of renal allo- of temsirolimus. Temsirolimus/FK506-binding protein affects graft rejection. Laboratory studies of rapamycin starting in the only one subpopulation of mTOR proteins, which reside in a early 1980s showed antitumor effects in several solid tumors. multiprotein complex termed mTORC1. An additional com- Cell cycle inhibitor-779 (now known as temsirolimus), a deri- plex, mTORC2, holds mTOR in a form that cannot be inhibited vative of rapamycin, was identified in the 1990s and subse- by temsirolimus and could have downstream signaling impli- quently developed as an anticancer agent (2). cations despite temsirolimus administration. The biology of mTOR, also known as rapamycin-associated protein, rapa- the mTOR signaling pathway and relevance to cancer has been mycin target, or sirolimus effector protein, is a molecule im- extensively reviewed (4, 5). Abnormalities in various compo- plicated in multiple tumor-promoting intracellular signaling nents of the mTOR pathway have been described for a wide pathways (Fig. 1). mTOR is a 289-kDa serine/threonine-specific variety of sporadic malignancies. In addition, mutations that kinase with highly conserved structure. It exists in cytoplasm in disrupt the tuberous sclerosis complex and other pathways a complex with three peptides: regulatory-associated protein of associated with inherited hamartoma syndromes also result in mTOR (raptor), mLST8, and GhL. Regulation of mTOR path- unregulated activation of mTOR (6). way activation is mediated through a series of complex sig- Temsirolimus showed antitumor effects across a wide naling interactions linking growth factor receptor signaling variety of tumor histotypes in preclinical models, particularly and other cell stimuli, phosphatidylinositol 3-kinase activation, those with defective PTEN (7–9). An initial dose-escalation and activation of the Akt/protein kinase B pathway. Two dis- phase I trial administered temsirolimus i.v. weekly at doses 2 tinct pathways have effects on cell cycle regulation downstream ranging from 7.5 to 220 mg/m (10). Although thrombocy- of mTOR. mTOR phosphorylates and activates p70 S6 kinase, topenia was dose limiting and reversible maculopapular rash leading to enhanced translation of certain ribosomal proteins and stomatitis were observed, the formal definition of a and elongation factors. This process leads, among other effects, maximum tolerated dose was not met. In addition, objective to the production of hypoxia-inducible factor-1a, which regu- partial and minor responses were observed at lower dose lates the transcription of genes that stimulate cell growth and levels. Based on these observations and pharmacokinetic data angiogenesis, including VEGF. Indeed, it has been shown that indicating comparable area under the curve variability mTOR inhibition leads to RCC cell line and xenograft tumor between body surface area–normalized and flat dosing, subsequent phase II testing investigated one or more of the following dose levels: 25, 75, and/or 250 mg i.v. weekly. Phase II trials were conducted in glioblastoma multiforme (11, 12), with well-documented PTEN alterations, mantle cell lymphoma (13), with potential mTOR-mediated cyclin D1 Author’s Affiliation: Department of SolidTumor Oncology and Urology, Cleveland ClinicTaussig Cancer Center, Cleveland, Ohio regulation, as well as melanoma (14), neuroendocrine tumors Received 11/19/07; revised 12/5/07; accepted 12/13/07. (15), breast cancer (16), and lung cancer (17). Modest single- Requests for reprints: Brian I. Rini, Department of Solid Tumor Oncology agent activity was observed in these trials of pretreated and Urology, Cleveland Clinic Taussig Cancer Center, 9500 Euclid Avenue/Desk patients, most notably in mantle cell lymphoma and breast R35, Cleveland, OH 44195. Phone: 216-444-9567; Fax: 216-636-1937; E-mail: [email protected]. cancer (Table 1). No evidence of a dose-response relationship F 2008 American Association for Cancer Research. was evident in any trial, and higher dose levels generally doi:10.1158/1078-0432.CCR-07-4719 resulted in greater toxicity.

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Fig. 1. mTOR signaling and temsirolimus. Cell stimulation through various means initiates an intracellular cascade that leads to the activation of the kinase mTOR, which has a key role in cell growth and proliferation, regulation of apoptosis, and angiogenesis.Temsirolimus binds to FK506-binding protein (FKBP), and the resultant protein-drug complex inhibits the kinase activity of the mTORC1complex. PI3-K, phosphatidylinositol 3-kinase; p70S6K, p70 S6 kinase; 4E-BP1, 4E-binding protein-1;eIF-4E, eukaryotic initiation factor-4 subunit E; HIFa, hypoxia-inducible factor a.

Temsirolimus was also tested in treatment-refractory, meta- treated patients in this poor-prognosis group had a median static RCC in a phase II trial that randomized 111 patients to overall survival of 8.2 months compared with 4.9 months for 25, 75, or 250 mg i.v. weekly (18). The overall response rate historical control IFN-a–treated patients (19). No evidence of was 7%, with an additional 26% of patients showing minor a dose response was seen in this trial. responses (Table 1). Retrospective assignment of risk criteria to A subsequent randomized phase III trial was conducted in patients in this study identified a poor-prognosis group with patients with metastatic RCC and three or more adverse risk three or more adverse risk features as established for metastatic features as defined by existing prognostic schema [the five RCC patients receiving IFN-based initial systemic therapy (19). noted above plus a sixth factor of three or more metastatic sites These risk factors consisted of Karnofsky performance status identified as prognostic in a separate analysis (20)]. A total <80%, lactate dehydrogenase >1.5Â laboratory upper limit of of 626 patients were randomized with equal probability to normal, hemoglobin < laboratory lower limit of normal, serum 25 mg temsirolimus i.v. weekly (n = 209) versus 18 million calcium corrected for albumin >10 mg/dL, and time from first units IFN-a thrice weekly (n = 207) versus 15 mg temsirolimus diagnosis of RCC to start of therapy of <1 year. Temsirolimus- i.v. weekly + 6 million units IFN-a thrice weekly (n = 210).

A separate phase I trial had established the safety maximum performance status V70%. Recent subset analysis from this trial tolerated dose of combination therapy with temsirolimus (21). supports a hypothesis of the greatest benefit of temsirolimus in The primary study end point of the phase III study was overall the poorest-risk patients and in patients with non–clear cell survival and the study was powered to compare each of the histologies (22). These results must be interpreted cautiously temsirolimus-containing arms with the IFN-a arm. Patients given small numbers and the retrospective nature but may treated with temsirolimus had a statistically longer overall identify the clinical phenotype associated with response to this survival than IFN-a monotherapy patients (10.9 versus 7.3 agent. The tumor biology of poor-risk and/or non–clear cell months; P = 0.0069; Table 1; ref. 1). There was also a RCC patients that may account for a greater effect of mTOR progression-free survival benefit from temsirolimus monother- inhibition is unclear at present. apy versus IFN (median, 3.8 versus 1.9 months; P < 0.0001). Toxicity of this agent is best evaluated from the prospective The objective response rate was 9% in the temsirolimus randomized RCC trial. Grade 3 adverse events occurred in 67% monotherapy arm. There was no survival advantage to the of the temsirolimus monotherapy group (lower than the other combination therapy arm over IFN-a monotherapy, perhaps two treatment arms). The most frequently occurring temsiroli- due to the lower dose of temsirolimus delivered, which may mus-related grade 3 or 4 hematologic toxicities included anemia have been inadequate for mTOR inhibition. These data lead to and thrombocytopenia. Hypercholesterolemia, hyperlipidemia Food and Drug Administration approval of temsirolimus for and hyperglycemia were also more common in the temsi- advanced RCC on May 31, 2007. Results from these studies rolimus arm, reflecting inhibition of mTOR-mediated lipid and validated mTOR as a relevant therapeutic target in RCC, at least glucose metabolism, and generally manageable with dietary or in the subset of patients with multiple adverse risk features. medical management. The most frequently occurring grade 3 Indeed, the study population was truly poor risk, with a third of adverse events in the temsirolimus arm were asthenia (11%), patients without prior nephrectomy and 80% with a Karnofsky anemia (20%), and dyspnea (9%). Dyspnea may be due in part

Table 1. Select clinical trial results of temsirolimus

Disease Trial design Clinical outcome RCC (1) Randomized phase III trial: temsirolimus ORR: 9% monotherapy vs IFN-a vs combination PFS: 3.7 mo (vs 1.9 mo for IFN-a temsirolimus/IFN-a in poor-risk metastatic monotherapy arm; P = 0.0001) RCC patients (n = 626) OS: 10.9 mo (vs. 7.3 mo for IFN-a monotherapy arm; P = 0.0069)

RCC (18) Randomized phase II: 25, 75, or 250 mg i.v. ORR: 7% weekly in treatment-refractory patients PFS: 5.8 mo (n = 111)

Mantle cell lymphoma (13) Phase II in relapsed/refractory patients: ORR: 38% 250 mg i.v. weekly (n = 35) PFS: 6.5 mo Duration of response: 6.9 mo

Breast cancer (16) Randomized phase II: 75 or 250 mg i.v. ORR: 9% weekly in treatment-refractory patients PFS: 12 wk (1-2 prior regimens; n = 109) Duration of response: 6 mo

Extensive-stage small cell lung Randomized phase II: 25 or 250 mg i.v. ORR: 1.2% cancer (17) weekly in stable/responding patients after PFS: 2.2 mo induction chemotherapy (n = 87)

Melanoma (14) Phase II trial of 25 mg i.v. weekly in treatment- ORR: 3% refractory patients (1-2 prior biotherapy/ PFS: 10 wk chemotherapy regimens; n = 33)

Neuroendocrine carcinoma (15) Phase II trial of 25 mg i.v. weekly ORR: 5%; tumor burden reduction (0-4 prior regimens; n = 37) in 57% of patients PFS: 6 mo

Glioblastoma (12) Phase II trial of 25 mg i.v. weekly ORR: 0%; 36% of patients with (0-1 prior regimens; n = 65) regression* PFS: 2.3 mo

Glioblastoma (11) Phase II trial of 250 mg i.v. weekly ORR: 5% (radiation-refractory and 0-3 prior chemotherapy PFS: 9 wk regimens; n = 43)

Abbreviations: ORR, objective response rate; PFS, progression-free survival; OS, overall survival. *Regression defined as unequivocal reduction in size of contrast enhancement or decrease in mass effect.

Clin Cancer Res 2008;14(5) March 1, 2008 1288 www.aacrjournals.org Downloaded from clincancerres.aacrjournals.org on October 1, 2021. © 2008 American Association for Cancer Research. Temsirolimus, an Inhibitor of mTOR to an underlying treatment-induced pneumonitis. Pulmonary cal staining of paraffin-embedded RCC samples with demon- toxicity with sirolimus has been reported in the transplant stration of increased expression of phosphorylated Akt, mTOR, population as patchy or diffuse infiltrates accompanied by and S6 in the majority of samples (26–28). Target inhibition dyspnea/dry cough and either a restrictive pattern or reduced has been shown after temsirolimus administration by decreased diffusing capacity for carbon monoxide on pulmonary function p70 S6 kinase activity as assayed in peripheral blood mono- testing (23). The phase III RCC trial reported six patients (5%) nuclear cells (2) and inhibition of S6 phosphorylation as with nonspecific pneumonitis not related to dose. One retro- measured by immunohistochemistry of posttreatment neuro- spective review of 22 patients with endometrial cancer or neu- endocrine tumors (15). Molecular determinants of response roendocrine tumors enrolled on phase II trials of 25 mg have also been investigated. Baseline expression of phosphor- temsirolimus i.v. weekly identified 8 patients (36%) with ylated S6 (15, 29) or phosphorylated p70 S6 kinase (12) was radiographic changes on routine computed tomography scans identified as associated with objective response to temsirolimus (either ground glass opacification or parenchymal consolida- in small populations of patients. Further, a decrease in p70 S6 tion) consistent with possible drug-induced pneumonitis (24). kinase activity in paired pre- and post-temsirolimus peripheral Dyspnea and dry cough were observed in the 50% of patients blood mononuclear cells was associated with a longer time to who had clinical symptoms. No specific treatment was given, treatment failure (2). These translational efforts require expan- and temsirolimus treatment was continued in four patients sion and validation in large prospective sample sets but sup- without reported worsening of pulmonary abnormalities. The port target inhibition by temsirolimus and generate hypotheses exact incidence of radiographic changes consistent with pneu- about prediction of response. monitis, percentage of patients with clinically relevant symp- In summary, the mTOR pathway is likely critical across a toms, and the mechanism of this toxicity is poorly defined at broad spectrum of tumor types. Temsirolimus has shown anti- present. Immunosuppression is an additional potential toxicity tumor activity, most notably in poor-risk advanced RCC where a of temsirolimus given the known immunosuppressive effects of demonstration of overall survival benefit has been observed. A sirolimus. The phase III RCC trial, however, showed no randomized trial of RAD001 versus placebo in treatment- significant difference in the incidence of neutropenia, lympho- refractory RCC has completed accrual and results are pending. penia, or infection versus the IFN control arm (1). In addition, a The lack of significant antitumor effect of temsirolimus- limited analysis of lymphocyte subsets and proliferative mediated mTOR inhibition in some tumors, especially those responses to nonspecific antigens did not show any significant with predicted sensitivity based on alterations such as PTEN effect in temsirolimus-treated patients on a phase I trial (25). mutation, underscores the complex interplay of multiple signal- The weekly schedule of temsirolimus, in contrast to the daily ing pathways within a single tumor. Other rapamycin analogue schedule of sirolimus, may have implications for effect of inhibitors of mTOR have also begun clinical testing. Everolimus immune function. Additional experience in a greater number of (RAD001, Novartis) and AP23573 (Ariad Pharmaceuticals) patients and with longer-term treatment is required to fully have shown tolerability and preliminary antitumor activity in characterize the effect of temsirolimus on immune variables and small trials (30, 31). Potential differences in antitumor effect or clinically relevant immune-mediated toxicity. tolerability among these agents due to route/schedule of admi- As with any targeted agent, the presence of a viable target, nistration, potency against mTOR, or other drug effects are not measurement of target inhibition, and defining the molecular well characterized at present. More potent or complete mTOR phenotype of susceptible versus resistant tumors are critical inhibition (e.g., through agents that inhibit both mTORC1 and goals to allow for rational drug development. Some initial mTORC2), inhibition of multiple signaling pathways simul- efforts in this regard relevant to the mTOR pathway and taneously, and/or more precise molecular phenotyping of temsirolimus have been made. Evidence of mTOR pathway tumors to define mTOR pathway reliance are needed to build activation has been investigated through immunohistochemi- on the clinical benefits of temsirolimus observed to date.

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Brian I. Rini

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